Johanna M. Resig

Transfer of Nazca Ridge Pelagic Sediments to the Peru Continental Margin

A complex set of lithologies, including calcareous oozes, hemipelagites, and turbidites, was recovered from the landward wall of the Peru Trench at its intersection with the Nazca Ridge. The sequence occurs at a water depth of 4,900 m and overlies an acoustic basement in the lowermost continental slope. Early Pliocene calcareous ooze overlies Pliocene to Quaternary ooze; both of these deposits are sandwiched between late Pleistocene (⩽ 400,000 yr), organic-rich turbidites and hemipelagic deposits typical of the Peru Trench and margin. Planktonic and benthic foraminiferal assemblages indicate that the early Pliocene ooze originally was deposited on the Nazca Ridge above the calcium carbonate compensation depth (4,000 m) and to the west of the cool Peru-Chile Current. The Pliocene-Pleistocene ooze contains a temperate fauna associated with the Peru-Chile Current. Block faulting at the terminus of the Nazca Ridge displaced the calcareous ooze 1,900 m from the top of the ridge to the trench below. Apparently these lithologies were then folded against or thrust beneath the lower continental slope within the past 400,000 yr. The stratigraphic sequence and the physiographic setting of the Nazca Ridge–Peru Trench intersection indicate convergence of the Nazca Ridge with the South American block. A minimum convergence rate of 0.8 cm/yr is calculated for the Pleistocene based upon the past and present geographic positions of the calcareous ooze. The best estimate of the rate is 2.8 cm/yr.

Uplifted Turbidite Basins on the Seaward Wall of the Peru Trench

Hemipelagic sediment layers inter-bedded with silt turbidite units were recovered from an elongate basin seaward of the axis of the Peru Trench. The basin is elevated 700 m above the shallowest depth of the present trench axis between lat 7°20′ and 8°30′ S. Micro-faunal content, mineralogy, and topography point to a source for this turbidite on the continental shelf to the northeast of the basin. A radiocarbon date of 5,100 B.P. was obtained for the youngest turbidite in the core, which dates the beginning of uplift in the basin. Although the nature of vertical motion is difficult to determine, the data suggest that there has been a combination of uplift of the seaward wall of the trench and subsidence of the trench axis. These rapid vertical movements may be a consequence of rupturing and decoupling of the Nazca plate into two segments that are being subducted at differential rates or different angles. If the seaward wall is under compressional stress as we suggest, we have an apparent contradiction with earthquake first motions, which indicate extensional stress on the seaward wall of this and other trenches.

Age and preservation of Amphistegina (foraminifera) in Hawaiian beach sand: implication for sand turnover rate and resource renewal

Radiocarbon dating of tests of Amphistegina spp. from the surface sand of Hawaiian beaches has revealed ages up to 1500 years or more. Preservation is related to age: the residual umbos are older than the intact tests. The antiquity of some of the tests suggests that predictions of a sand turnover rate of less than 100 years time based on Amphistegina productivity studies are in error and that the majority of tests produced in nearshore environments of Hawaii do not accumulate on the beach.

Stratigraphic Distribution of Late Neogene Species of the Planktonic Foraminifer Streptochilus in the Indo-Pacific

Study of four species of the biserial planktonic foraminifer Streptochilus from Deep Sea Drilling Project cores of the Eauripik Rise, western equatorial Pacific, and Ninetyeast Ridge, Indian Ocean, shows that both the stratigraphic distribution of species and their frequency patterns (though not actual frequencies or abundances) are correlative in the two areas, supporting their use as stratigraphic and paleoecologic index fossils. Their distributional trends are linked to eustatic sea level changes and to changes in the mixing of surface waters; low frequencies and species turnovers occur during regressive phases when strong circulation of oxygenated waters could lead to the subsequent decline of their oxygen-minimum habitat. The species S. subglobigerum, S. latum, S. globigerum, and S. globulosum succeed one another at intervals averaging 2.5 my from late middle Miocene Zone N 15 through Quaternary Zone N23. The new species, Streptochilus subglobigerum, is described for what was formerly thought to be a stratigraphically lower, disjunct part of the range of S. globigerum. These four species most likely belong to a single phylogenetic lineage as evidenced by some transitional morphologies.

Pliocene-Holocene benthic foraminiferal assemblages and water mass history, ODP 806B, western Equatorial Pacific

A succession of benthic foraminiferal assemblages, defined through cluster analysis and characterized by high percent frequency of select members of the Pacific Deep and Bottom Water fauna, is recorded for the Pliocene to Holocene section of the Ontong Java Plateau. These data confirm an early Pliocene migration of Nuttallides umbonifera, an AABW-associated species, onto the surface of the plateau, as reported by Hermelin in 1989, and suggest enhanced production of bottom water between about 4.1 and 4.7 Ma. Uvigerina percent representation generally follows the trend in total organic carbon and in benthic foraminiferal abundance. The assemblage dominated by Uvigerina is recurrent in two parts of the midto upper Pliocene section that might be comparable to the deep oxygen minimum environment presently in place on the upper surface of the plateau. High percentages of Uvigerina in the Pleistocene section are overshadowed by the dominant species, Alabaminella weddellensis and Pseudoparrella exigua, opportunistic species that have been associated with phytodetritus. Alabaminella weddellensis, a small species, increases dramatically after the Pliocene/Pleistocene boundary. This diagnostic species has been overlooked in previous studies of the plateaus strata in which the very fine sand fraction was not inspected.

Foraminiferal stratigraphy and depositional history in the area of the Ontong Java Plateau

Abstract Foraminifera from 54 cores from the surface and slopes of the Ontong Java Plateau were used to identify Tertiary outcrops as old as Late Eocene on some of the slopes. Using the direct relationship between radiolarian concentrations and bathymetry, it was determined that the Tertiary deposits from the slopes accumulated in deeper water than the synchronous deposits sampled through the plateau surface (Deep Sea Drilling Project, Site 64), indicating that the topographic high has existed at least since early Tertiary time. Comparison of assemblages from several foraminiferal zones of the plateau and certain of the adjacent Solomon Islands supports the conclusion of a genetic connection between those islands and the plateau.

Middle Eocene pelagic microfossils from the Nazca Plate

A piston core of the oldest sediment yet recovered from the Nazca Plate was retrieved from the outer ridge of the Peru-Chile Trench, below the present calcite compensation level (CCD). The core, PCOD-19, contains assemblages of nanno-fossils of the upper middle Eocene Discoaster saipanensis Subzone, whereas the foraminifera are assigned to earlier zones of the middle Eocene, suggesting reworking. We suggest that these fossils, as well as basalt pebbles in the core, indicate that normal pelagic sedimentation was interrupted and that tectonic events on the sea floor are in part responsible for the disturbances in obducted stratigraphic sequences.

EVIDENCE OF LATE PLIOCENE–EARLY PLEISTOCENE MARINE ENVIRONMENTS IN THE DEEP SUBSURFACE OF THE LIHUE BASIN, KAUAI, HAWAII

Abstract Cuttings recovered from two deep exploratory wells in the Lihue Basin, Kauai, Hawaii, include fossiliferous marine deposits that offer an uncommon opportunity to study paleoenvironments from the deep subsurface in Hawaii and interpret the paleogeography and geologic history of Kauai. These deposits indicate that two marine incursions gave rise to protected shallow-water, low-energy embayments in the southern part of the Lihue Basin in the late Pliocene–early Pleistocene. During the first marine incursion, the embayment was initially zoned, with a variable-salinity environment nearshore and a normal-marine reef environment offshore. The offshore reef environment eventually evolved to a nearshore, variable-salinity environment as the outer part of the embayment shallowed. During the second marine incursion, the embayment had normal-marine to hypersaline conditions, which constitute a significant departure from the variable-salinity environment present during the first marine incursion. Large streams draining the southern Lihue Basin are a likely source of the freshwater that caused the salinity fluctuations evident in the fossils from the first marine incursion. Subsequent volcanic eruptions produced lava flows that buried the embayment and probably diverted much of the stream flow in the southern Lihue Basin northward, to its present point of discharge north of Kalepa Ridge. As a result, the embayment that formed during the second marine incursion received less freshwater, and a normal-marine to hypersaline environment developed. The shallow-water marine deposits, currently buried between 86 m and 185 m below present sea level, have implications for regional tectonics and global eustasy.